1. Field of the Invention
This invention relates to a non-volatile storage device, and more particularly to a non-volatile storage device which is reloadable by a low voltage.
Non-volatile memories are devices which are important in widening the application of general purpose LSIs; for example, they are used as PROMs (Programmable Read Only Memory).
FIGS. 5A and 5B of the accompanying drawings show a MNOS (Metal Nitride Oxide Semiconductor) memory as a typical non-volatile storage device. The MNOS memory comprises a silicon oxide film 12, a silicon nitride film 14 and an aluminum electrode 16, which are laminated in order on a silicon substrate 10 on which a drain region and a source region are formed. Although the silicon oxide film 12 causes a current normally not to flow, a current would flow due to a tunnel phenomenon if the silicon oxide film 12 is very thin and if a high voltage is applied to it. When this current is received by the silicon nitride film 14, charges would be captured from near the interface between the silicon nitride 14 and the silicon oxide film 12 toward the inside of the silicon nitride film 14. With the charges accumulated, a drain current does not flow unless a voltage sufficiently excessive to cancel the captured charges is applied, and the captured charges disappear even if no voltage is applied. Therefore the condition whether this drain current flows or does not flow corresponds to 1 bit of data.
Specifically, during storing data, as shown in FIG. 5A, a voltage higher than 5 V (usually about 25 V) is applied to the gate electrode to cause charges to be captured by the silicon nitride film 14, and during reading data, as shown in FIG. 5B, a voltage of 5 V is applied to the gate electrode and the drain electrode to detect whether or not a drain current flows.
With the conventional MNOS memory, although it is possible to electrically store and erase data, it requires a storing voltage of 25 V, which is relatively high.
To this end, a MONOS (Metal Oxide Nitride Oxide Semiconductor) memory which is an improvement of the MNOS memory and which requires only a storing voltage of less than 25 V has been proposed. As shown in FIG. 6, this MONOS memory comprises a silicon oxide film 12, a silicon nitride film 14, a silicon oxide film 15 and a polysilicon electrode 16, which are laminated in order on a silicon substrate 10 on which a drain region and a source region are formed. Thus with the silicon oxide film 15 between the silicon nitride film 14 and the polysilicon electrode 16, potential barriers will be formed at opposite side of the silicon nitride film 14 so that charges injected from the silicon substrate are captured with an improved efficiency. Since it is possible to eliminate the limit of reducing the thickness of the silicon nitride film 14, further reduction of the thickness of the silicon nitride film can be progressed to enable a low-voltage operation. Reducing the sum of captured charges by a very small thickness of the silicon nitride film cannot be neglected, and it is difficult to obtain an excellent characteristic due to non-stable operation even if the thickness of the silicon nitride film is optimum.